The present invention relates to satellite communications systems where a user terminal is able to communicate with an earth station via a communications satellite. In greater detail, the present invention relates to such communications systems where the user terminal utilises a paging channel in an attempt to gain communications via the satellite. In greatest particularity, the present invention relates to such systems where there may be more than one paging channel available, either from nearby spot beams from the same satellite or from more than one satellite.
In terrestrial based cellular telephone systems, the user terminal, usually in the form of a radio telephone handset, communicates with one or more spaced base stations. Each base station puts out broadcast messages on a broadcast channel (BCCH) and also provides a paging channel (PCH). The timing of the messages on each BCCH from the plurality of base stations and for each PCH is such that there is no opportunity for the BCCH from one base station to collide with the BCCH or PCH from another base station, nor any opportunity for the PCH of one base station to collide with the PCH or BCCH of any other base station. Thus, if a user terminal, in a terrestrial cellular system, wishes to decode paging from a neighbouring base station, it is required to listen to the BCCH of the neighbouring base station to learn the PCH details of the neighbouring base station, and is then able to access the PCH of the neighbouring base station without fear of collision between BCH and PCH channels. Collision, where one signal coincides in time with another, renders signals unreadable and the vital information is lost.
Satellite communication systems generally resemble cellular systems in that a user terminal communicates with a base station in the form of an earth station. However, the presence of an intervening communications satellite complicates the issue. The communications satellite receives messages from the earth station and relays them to the user terminal. The communications satellite also receives messages from the user terminal and relays them to the earth station. Unfortunately, while a terrestrial cellular system has virtually no propagation delay between the base station and the user terminal, the high flying altitude of the communications satellite and separation on the surface of the earth between the user terminal and the earth station means that a significant propagation delay ensues. In the example to be given in the description of the preferred embodiment, round trip (earth-satellite-earth) propagation delays can exceed one third of a second.
Communications satellites provide the user terminal a plurality of spot beams in a spaced array of areas of radio coverage on the surface of the earth. While these correspond, in general, to the cells of the terrestrial cellular systems, there are major differences. The spot beams are much larger and significant differences in propagation delays can result between adjacent and nearby beams. So that the coverage should have no gaps, the spot beams overlap to a considerable extent, resulting in the signals for one spot beam being audible in many others, and vice versa. Thus, even if an attempt is made, as in a terrestrial system, to synchronise the BCCH and PCH channels for each spot beam over the entire range of the array of spot beams, so that no collision occurs, there is still a significant chance that the differential propagation delays will cause collisions within the spot beams of a single satellite.
Even worse, the communications satellite is generally one of a plurality of communications satellites in orbit about the earth in a constellation which may include a plurality of overlapping planes. The cones of radio coverage, each divided into spot beams, are forced to display overlap so that no portion of the surface of the earth is left without satellite coverage. Thus, spot beams from different satellites in the same orbital plane overlap, and spot beams from satellites in different orbital planes overlap. A user terminal can thus find itself subject to signals from two, three, or more satellite at the same time. Again, because of differential propagation delays between the different satellites, attempts at BCCH and PCH synchronicity are doomed to fail, and if no synchronicity is attempted between satellites, there will be inevitable collisions.
The nature and origin of these problems is further explained in the description of the preferred embodiment, provide hereafter.
Thus, current art techniques are of no avail to overcome these difficulties. It is possible to provide incredibly elaborate schemes for inter-spot-beam and inter-satellite synchronicity, but these would impose a very high equipment and system time overhead, as well as increasing the system cost very considerably by rendering the user terminals, which could be numbered in millions, each considerably more elaborate than is currently necessary. What is required is a simple approach which can overcome all of these problems.
The present invention seeks to provide such a solution.
According to a first aspect, the present invention consists in a method for use in a satellite communications system, employing a concatenation of timeslots for transmission or reception, a user terminal being operative to select an individual timeslots from a each of a repetitive plurality of spaced patterns of timeslots as a paging channel, said method being characterised by said user terminal periodically, with a first period, altering the selected timeslot in each pattern for the selected timeslot to execute a cycle through each of the timeslots in each pattern.
According to a second aspect, the present invention consists in a user terminal for use in a satellite communications system employing a concatenation of timeslots for transmission or reception, said user terminal being operative to select an individual timeslots from a each of a repetitive plurality of spaced patterns of timeslots as a paging channel, said user terminal being characterised by being operative, periodically, with a first period, to alter the selected timeslot in each pattern for the selected timeslot to execute a cycle through each of the timeslots in each pattern.
According to a third aspect, the present invention consists in a satellite communications system a including a user terminal and employing a concatenation of timeslots for transmission or reception, a user terminal being operative to select an individual timeslots from a each of a repetitive plurality of spaced patterns of timeslots as a paging channel, said system being characterised by said user terminal being operative, periodically, with a first period, to alter the selected timeslot in each pattern for the selected timeslot to execute a cycle through each of the timeslots in each pattern.
The first, second and third aspects also provide for the periodic altering of the selected timeslot including alteration after each pattern.
The first, second and third aspects also provide that the concatenation of timeslots includes a plurality of equispaced timeslots bearing broadcast bursts, the broadcast bursts separating and internally dividing the patterns of timeslots, the broadcast bursts having a repetition rate which is an integral multiple of the repetition rate of the patterns, and each of the broadcast bursts defining a timeslot which is not in any of the patterns of timeslots, consecutive integral numbers of bursts carrying a message structure which has a length of consecutive bursts which is selected from a set of predetermined numbers of consecutive broadcast bursts, and that user terminal, periodically, with a second period, alters the selection of the selected timeslot one place further in the cycle, the second period being a consecutive repetition number of broadcast bursts, the repetition number not being in the set of predetermined numbers and being neither divisible by nor divisible into any member of the set.
The first, second and third aspects, yet further, provide that the user terminal calculates a paging channel group, counts the timeslots to calculate an incremental paging group number, uses the incremental paging group number to calculate a timeslot allocation number, and counts the timeslots in each pattern and employs the timeslot allocation number to select the timeslot for said paging channel.
According to a fourth aspect, the present invention consists in a method for use in a satellite communications system including the steps of employing a user terminal to receive broadcast message frames and to select a paging channel at a predetermined time after commencement of said broadcast message frames, said method being characterised by including the step of said user terminal periodically, progressively and cyclically, altering said predetermined time.
According to a fifth aspect, the present invention consists in a user terminal for use in a satellite communications system, operative to receive broadcast message frames and to select a paging channel at a predetermined time after commencement of said broadcast message frames, said user terminal being characterised by being further operative, periodically, progressively and cyclically, to alter said predetermined time.
According to a sixth aspect, the present invention consists in a satellite communications system including a user terminal, operative to receive broadcast message frames and to select a paging channel at a predetermined time after commencement of said broadcast message frames, said system being characterised by said user terminal being operative, periodically, progressively and cyclically, to alter said predetermined time.